1. Field of the Invention
Keeping step with recent innovations and developments in machine technology, bearings employed in various machine parts are subjected to ever severer usage conditions. In particular, bearings used in automotive electrical components such as alternators, electromagnetic clutches and idler pulleys are exposed to very severe usage conditions, including higher temperatures due to improvements in the performance of engines as heat-generating elements, higher speeds due to improvements in the performance of parts and higher loads due to increases in belt tension. These bearings also face the problem of rusting due to penetration of salt water or sea water. The present invention relates to a lubricating grease composition useful in such high-temperature, high-speed and high-load bearings.
2. Description of the Related Art
Greases which have heretofore been employed in such high-temperature, high-speed and high-load bearings are those using, as a thickener, a diurea compound containing cyclohexyl groups as predominant end groups and, as a base oil, a poly-.alpha.-olefin lubricant.
With the ever severer usage conditions, such conventional diurea greases containing cyclohexyl groups as predominant end groups are prematurely softened under high-temperature and high-speed conditions so that their leakage from bearings increase. Moreover, the poly-.alpha.-olefin lubricant is close to its maximum in heat resistance and oxidative stability so that it has become difficult to obtain satisfactory lubricating life with greases containing the lubricant as a base oil. In addition, the conventional greases involve the problem that bearings undergo premature flaking. Effective for the improvement of these problems are greases which use, as a thickener, a diurea compound containing aromatic groups as predominant end groups out of those disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 1-259097 and, as a base oil, an alkyldiphenyl ether lubricant. Greases making use of this thickener are, however, accompanied by the problems that they are inferior in fluidity and, under high load conditions, no sufficient greases are allowed to flow into lubricated parts and bearings are seized prematurely.